Vitamins are a very important part of our diet. These are substances that our body requires for the normal development of our system. As our body cannot synthesize these organic micronutrients, we can only get them from food sources. There are about 15 vitamins essential for humans. They are classified as fat-soluble vitamins and water-soluble vitamins.
Fat-soluble vitamins are A, D, E, and K whereas water-soluble vitamins include the B group and vitamin C. Most of the water-soluble vitamins exert functions through their respective coenzyme while only one fat-soluble vitamin (Vitamin K) can function as a coenzyme.
What are Fat-soluble Vitamins?
Fat-soluble vitamins are stored in the body’s fatty tissues and liver, and they play a critical role in various bodily functions. Our body can store these vitamins for long period of time.
Further, fat-soluble vitamins are not as readily absorbed from the diet as water-soluble vitamins and are eliminated much more slowly than water-soluble vitamins. But the good thing about fat-soluble vitamins is that the body does not need these vitamins every day and stores them in the liver and adipose tissue when not used. The fat-soluble vitamins include vitamins A, D, E, and K.
We should keep in mind that if we over consume these vitamins, they may cause toxicity.
The Role of Fat-Soluble Vitamins
- Vitamin A: This vitamin is renowned for its role in maintaining healthy vision and promoting proper immune system function. It also contributes to skin health and the growth and development of various body tissues.
- Vitamin D: It is crucial for calcium absorption and bone health. It also plays a role in supporting immune function and reducing inflammation.
- Vitamin E: It is a potent antioxidant. Vitamin E helps protect cells from damage caused by free radicals. It also plays a role in immune function and skin health.
- Vitamin K: This vitamin is essential for blood clotting and bone health. It aids in the synthesis of proteins that are necessary for these vital processes.
Fat-soluble vitamins list
There are four Fat-soluble Vitamins:
- Retinol: Vitamin A
- Cholecalciferol: Vitamin D
- Tocopherol: Vitamin E
- Vitamin K
They do not act as a coenzyme like water-soluble vitamins (exception: vitamin K). Vitamin A & D act like hormones and can be toxic in excess.
Vitamin A
Chemical Name: Retinol
Other Names: Retinoids, retinal and retinoic acid
What is Retinol?
It is a fat-soluble vitamin essential for a variety of functions such as vision, hair growth, body growth, reproduction and maintenance of epithelial cells. In addition, it also plays an important role in the regulation of the immune system.
Retinal and retinoic acid are the active forms of vitamin A. It is only present in foods of animal origin. However, its provitamins carotenes are present in plants.
Structure of Vitamin A
Vitamin A represents many structurally and biologically related molecules such as retinal, retinoic acid and retinal.
Let’s discuss their structure.
Retinol
Primarily, it is an alcohol that contains a β-ionone ring. The side chains have two isoprenoid units, four double bonds and one hydroxyl group.
Retinal
The Retinal is an aldehyde. It results from the oxidation of retinol by the enzyme retinal reductase. This reaction is readily reversible. Hence, retinal and retinol are interconvertible compounds.
Retinoic acid
Further, it is obtained by the oxidation of the retinal. However, this reaction is not reversible. Thus, retinoic acid cannot be converted back to retinal or retinol.
Food sources of Vitamin A
Non-vegetarian sources: Liver, kidney, egg yolk, fish liver oils (cod or shark)
Vegetarian sources: green leafy vegetables and fruits such as mango and papaya.
Also, vegetable sources contain the yellow pigment beta carotene. Carrot contains a significant quantity of β-carotene
Sources such as green leafy vegetables and fruits are good sources of carotenes.
Vegetables such as spinach, carrots, amaranthus, and pumpkins contain beta carotenes.
Daily recommended Intake of Vitamin A
The recommendation for vitamin A intake is expressed as micrograms (mcg) of retinol activity equivalents (RAE). Retinol activity equivalents account for the fact that the body converts only a portion of beta-carotene to retinol. One RAE equals 1 mcg of retinol or 12 mcg of beta-carotene. The Recommended Dietary Allowance (RDA) for vitamin A is 900 mcg/ day for adult males and 700 mcg/day for adult females.
The requirement increases in growing children, pregnant and lactating women. Studies indicate that vitamin A requirements may increase due to hyperthyroidism, fever, infection, cold, and exposure to excessive amounts of sunlight. Those who consume excess alcohol or have the renal disease should also increase their intake of vitamin A.
| Age | Recommended Intake of Vitamin A (mcg/day) |
|
|---|---|---|
| INFANTS | 0-6 M | 400 |
| 7-12 M | 500 | |
| CHILDREN | 1-3 Yr | 300 |
| 4-8 Yr | 400 | |
| 9-13 Yr | 600 | |
| FEMALE | 18+ Yr | 700 |
| MALE | 18+ Yr | 900 |
| PREGNANCY | 750-770 | |
| LACTATION | 1200-1300 |
Vitamin A Deficiency
Vitamin A deficiency may occur due to:
- Poor intake
- Jaundice that can cause poor absorption
- Severe malnutrition
- Chronic nephrosis
Vitamin A deficiency symptoms are visible on the eyes and skin. Although, the deficiency symptoms are not immediate as the hepatic stores can meet the body requirements for about 2-4 months.
Severe vitamin A deficiency occurs mostly in developing countries but it is also fairly common in patients with severe liver disease or fat malabsorption. Pregnant and lactating women are also a victim of vitamin A deficiency. The
breastfed children of mothers who themselves are vitamin A deficient are prone to vitamin A deficiency.
Below is a list of vitamin A deficiencies:
1. Night Blindness (Nyctalopia)
Vitamin A deficiency presents as defective night vision or night blindness. It is one of the earliest symptoms of vitamin A deficiency. The person has difficulty seeing in dim light since the dark adaptation time is increased.
2. Xerophthalmia
Severe deficiency may result in xerophthalmia. Its symptoms include dry, thick and wrinkled conjunctiva. The conjunctiva also gets keratinized and loses its normal transparency.
Conjunctiva is a membrane of the eye that covers most of the visible parts of the eye.
3. Bitot’s Spots
These are greyish-white triangular plaques on the white surface of the eyeball. This is due to the increased thickness of the conjunctiva in certain areas.
The symptoms include itchy or burning eyes and decreased vision. The person will also experience sudden episodes of blurredness lasting for a few minutes.
4. Keratomalacia
When the xerophthalmia persists for a long time, it progresses to keratomalacia. This is an eye condition in which the cornea gets cloudy and softens. If Keratomalacia is not treated, the softening of the cornea can lead to infection, rupture, and also tissue changes. The bacterial infection further leads to corneal ulceration, perforation of the cornea and total blindness.
5. Other menifestations of Vitamin A deficiencies
Vitamin A deficiency results in growth retardation due to impairment in skeletal formations. It also has an adverse effect on the reproductive system. Further, the skin may become dry and rough. It also affects the gastrointestinal tract, urinary tract and epithelial tract.
Vitamin A Toxicity
Overconsumption of Vitamin A may cause toxicity. The symptoms of vitamin A toxicity (hypervitaminosis A) include dermatitis, enlargement of the liver, skeletal decalcification, tenderness of long bones, loss of weight, irritability, loss of hair, joint pain, peeling of the skin, drowsiness. vomiting, etc.
A higher concentration of vitamin A increases the synthesis of lysosomal hydrolases, leading to cellular death.
2. Vitamin D:
Chemical Name: calciferol, cholecalciferol (D3), Ergocalciferol (D2)
Other Name: Sun-shine vitamin
Vitamin D is a fat-soluble vitamin that resembles sterols in structure and functions. It plays a very crucial role in the maintenance and formation of bones. Calcitriol is the biologically active form of vitamin D. It regulates the plasma levels of calcium and phosphates. Calcitriol increases the intestinal absorption of calcium and phosphate. It is also involved in minimizing the excretion of calcium and phosphate through the kidney.
Vitamin D benefits the body by playing a role in immunity and controlling cell growth and may protect against osteoporosis, high blood pressure, cancer, and other diseases.
Structure of vitamin D
Vitamin D is a group of closely related sterols produced by the action of ultraviolet light (wavelength 290–310 nm) on
provitamins. Ergocalciferol will form from ergosterol and is present in plants whereas cholecalciferol is present in animals. On exposure to sunlight, 7-dehydrocholesterol (an intermediate of cholesterol synthesis) will convert to cholecalciferol in the skin. Ergocalciferol and cholecalciferol are both reffered to as provitamins.
Food Sources of Vitamin D
Non-vegetarian Sources: fish liver oils, egg yolk, fatty fish, salmon.
Vegetarian sources: mushrooms when exposed to ultraviolet light may produce a significant amount of Vitamin D.
Milk and other fortified dairy products also contain vitamin D. However, milk is not a good source of vitamin D. In addition, our body can also synthesize vitamin D when exposed to sunlight.
Daily Recommended Dose of Vitamin D
The table below shows the daily recommended dose of vitamin D in all age groups.
| Age | Recommended Intake of Vitamin D (mcg/day) |
|
|---|---|---|
| INFANTS | 0-6 M | 10 |
| 7-12 M | 10 | |
| CHILDREN | 1-3 Yr | 15 |
| 4-8 Yr | 15 | |
| 9-13 Yr | 15 | |
| FEMALE | 18+ Yr | 15 |
| MALE | 18+ Yr | 15 |
| PREGNANCY | 15 | |
| LACTATION | 15 |
Vitamin D Deficiency
Relatively, its deficiency is less common, since the vitamin can be synthesized in the body. Although, insufficient exposure to sunlight and poor diet may result in its deficiency.
As the major sources of vitamin D are of animal origin, its deficiency occurs mostly in strict vegetarians. In addition, it may also develop in individuals with fat malabsorption or kidney and liver diseases.
The deficiency diseases are rickets in children and osteomalacia in adults.
1. Rickets
Rickets is derived from an old English word ‘wrickken’ that means “twist”. It develops in children. The classical features of rickets are bone deformities. In this disease, bones become soft and pliable. Tooth decay is also seen in its patients. In addition, the weight-bearing bones will bend to form bow-legs. In rickets, the plasma level of vitamin D will decrease and alkaline phosphatase activity will elevate.
The symptoms of rickets include bow legs, knock-knee, rickety rosary, bossing of frontal bones, and pigeon chest.
2. Osteomalacia
The term is derived from Greek “osteon” meaning bone, and “malakia” which means softness. In Osteomalacia, demineralization of the bones occurs. As a result, such people are more prone to get fractures.
Vitamin D Toxicity
Among vitamins, Vitamin D toxicity is the most toxic in overdoses. Symptoms include weakness, polyuria, intense thirst, difficulty in speaking, hypertension and weight loss. Although vitamin D is toxic in higher doses, excessive exposure to sunlight does not result in vitamin D toxicity. It is because excess vitamin D3 is destroyed by sunlight itself.
The Tolerable Upper Intake Level (UL) for vitamin D is set at 4000 IUs for people 9 years of age and older. High doses of vitamin D supplements coupled with large amounts of fortified foods may cause accumulations in the liver and produce signs of poisoning.
Vitamin E
Chemical Name: Tocopherol
Other Name: Anti-sterility Vitamin
What is Vitamin E?
It is a naturally occurring antioxidant that also protects vitamins A and C, red blood cells, and essential fatty acids from destruction. In addition, this vitamin is essential for normal reproduction in animals. hence, it is known as an anti-sterility vitamin. Vitamin E also boosts immune response.
Structure of Vitamin E
A chromane ring system, with an isoprenoid side chain, is present in all the eight naturally occurring tocopherols. Among these, alpha-tocopherol (5, 7, 8-trimethyl tocol) has the greatest biological activity. The antioxidant property is due to the chromane ring.
Sources of Vitamin E
Non-vegetarian sources: Meat, eggs.
Vegetarian Sources: vegetable oils (wheat germ oil, sunflower oil, safflower oil, cottonseed oil), avocados, peanut butter, almonds.
Vitamin E sources also include fruits and vegetables, grains, and nuts.
Daily Recommended Dose of Vitamin E
The Recommended Dietary Allowance (RDA) for vitamin E is based on the most active and usable form called alpha-tocopherol. RDA guidelines state that males and females over the age of 14 should receive 15 mcg (22.5 IUs) of alpha-tocopherol per day. However, consuming vitamin E in excess does not result in any added benefits.
| Age | Recommended Intake of Vitamin E (mg/day) |
|
|---|---|---|
| INFANTS | 0-6 M | 4 |
| 7-12 M | 5 | |
| CHILDREN | 1-3 Yr | 6 |
| 4-8 Yr | 7 | |
| 9-13 Yr | 11 | |
| FEMALE | 18+ Yr | 15 |
| MALE | 18+ Yr | 15 |
| PREGNANCY | 15 | |
| LACTATION | 19 |
Vitamin E Deficiencies
Vitamin E deficiency symptoms vary from one animal species to another. Some of the symptoms include degenerative changes in muscle, megaloblastic anaemia, changes in the central nervous system, and sterility. In rats, inability to produce healthy ovum and loss of motility of spermatozoa, hemolysis of red cells, acute hepatic necrosis and muscular dystrophy are observed.
However, in humans, cases of vitamin E deficiency is very uncommon. In severe cases, minor neurological symptoms are observed.
Vitamin E Toxicity
Vitamin E is the least toxic among fat-soluble vitamins. The toxicity of vitamin E is difficult when it is consumed from natural dietary sources. No toxic effects have been reported even after ingestion of 300mg/day.
Vitamin K
Chemical Name: phylloquinone (K1), menaquinone (K2)
Other Name: None
Vitamin K plays an essential role in normal blood clotting as well as in promoting bone health. In addition, it is the only fat-soluble vitamin that has a specific coenzyme function.
Structure
Vitamin K exists in different forms:
- K1 (phylloquinone): present in plants.
- K2 (menaqquinone): produced by the intestinal bacteria and also found in animals.
- K3(menadione): it is a synthetic form.
These are similar in structure because they share a quinone ring but differ in the length and degree of saturation of the carbon tail and the number of repeating isoprene units in the side chain. All three forms are heat-stable. However, they can lose their activity by oxidizing agents, irradiation, strong acids, and alkalis.
Food sources for vitamin K
Non-vegetarian sources: Egg yolk, meat, liver,
Vegetarian sources: Cabbage, cauliflower, parsley, kale, tomatoes, spinach, and other green vegetables.
In addition, cheese and other dairy products also contain vitamin K.
The best dietary sources of vitamin K1 are leafy green vegetables, whereas vitamin K2 is mainly found in animal-sourced foods and fermented soy products. Moreover, plant-based sources have a higher content of vitamin K as compared to animal-based sources. Therefore, it is best to obtain vitamin K from green leafy vegetables.
Daily Recommended Dose of Vitamin K
The table below shows the daily recommended dose of vitamin K in all age groups.
| Age | Recommended Intake of Vitamin K (mcg/day) |
|
|---|---|---|
| INFANTS | 0-6 M | 2.0 |
| 7-12 M | 2.5 | |
| CHILDREN | 1-3 Yr | 30 |
| 4-8 Yr | 55 | |
| 9-13 Yr | ||
| FEMALE | 9-13 Yr | 60 |
| 14-18 Yr | 75 | |
| 19+ Yr | 90 | |
| MALE | 9-13 Yr | 60 |
| 14-18 Yr | 75 | |
| 19+ Yr | 120 | |
| PREGNANCY | 75-90 | |
| LACTATION | 75-90 |
Vitamin K Deficiency
Since it is present in sufficient quantity in diet, it is uncommon to have vitamin K deficiency. In addition, bacteria present in our intestine also produce an adequate amount of vitamin K. However, vitamin K deficiency may occur due to poor or faulty absorption of vitamin K.
People who do not efficiently digest and absorb fat are at the greatest risk of developing vitamin K deficiency. This includes patients with celiac disease, inflammatory bowel disease and also cystic fibrosis.
Vitamin K deficiency may result in:
- Hemorrhagic disease of the newborn: premature infants may develop vitamin K deciency. This is due to lack of hepatic stores, limited oral intake and absence of intestinal bacterial flora.
- Prolongation of prothrombin time as well as delayed clotting time.
- The individual bleeds profusely even for minor injuries.
Vitamin K Toxicity
Unlike the other fat-soluble vitamins, natural forms of vitamin K does not have known symptoms of toxicity.
References
Biochemistry, 4th edition by Dr U. Satyanarayana, Section one: chemical constituents of life, chapter no. 7: Vitamins, page no. 132-156
Textbook of Biochemistry For Medical Students, Sixth Edition by DM Vasudevan, chapter no. 34: fat-soluble vitamins, page no. 379-390.
Medical Biochemistry, 4th edition, John W. Baynes, PhD, chapter number 11: Vitamins and minerals-Fat-soluble vitamins.
Harper’s Illustrated Biochemistry, 28th edition, Robert K. Murray, David A Bender, Kathleen M. Botham, Peter J. Kennelly, Victor W. Rodwell, P. Anthony Weil, chapter number 44: Micronutrients- Vitamins and Minerals.
https://www.healthline.com/nutrition/fat-soluble-vitamins#vitA
Fat-Soluble Vitamins: A, D, E, and K – 9.315
https://www.healthline.com/health/keratomalacia
https://www.medicalnewstoday.com/articles/320310#vitamin-a